net: expand NETDEV_RSS_KEY_LEN to 256 bytes

NETDEV_RSS_KEY_LEN has been set to 52 bytes in 2014, until now.

Jakub suggested we bump the size to 128 bytes or more.

Some drivers (like idpf) were already working around the core limit.

Since this change might cause some issues in admin scripts,
bump it directly to 256 in one go.

tjbp26:~# cat /proc/sys/net/core/netdev_rss_key | wc -c
768

tjbp26:~# ethtool -x eth1
RX flow hash indirection table for eth1 with 32 RX ring(s):
...
RSS hash key:
fe:16:5b:2f:93:85:c2:c9:c1:ef:bd:60:c6:e0:2b:99:4d:bf:b7:14:c8:1e:8d:cb:31:17:51:da:55:eb:91:d9:9e:f9:89:9b:44:a1:dc:08:72:3a:b3:d6:31:86:9a:fe:02:3a:0d:eb:a1:7c:f5:a3:51:3b:08:56:c9:3f:71:69:01:ba:70:38
RSS hash function:
    toeplitz: on
    xor: off
    crc32: off

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/netdev/20260122075206.504ec591@kernel.org/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260122190349.2771064-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 16 insertions, 10 deletions

diff --git a/Documentation/admin-guide/sysctl/net.rst b/Documentation/admin-guide/sysctl/net.rst
index 91fa4ccd326c..19408da2390b 100644
--- a/Documentation/admin-guide/sysctl/net.rst
+++ b/Documentation/admin-guide/sysctl/net.rst
@@ -314,21 +314,22 @@ Default: 1000
 netdev_rss_key
 --------------
 
-RSS (Receive Side Scaling) enabled drivers use a 40 bytes host key that is
-randomly generated.
+RSS (Receive Side Scaling) enabled drivers use a host key that
+is randomly generated.
 Some user space might need to gather its content even if drivers do not
 provide ethtool -x support yet.
 
 ::
 
   myhost:~# cat /proc/sys/net/core/netdev_rss_key
-  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8: ... (52 bytes total)
+  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8: ... (256 bytes total)
 
-File contains nul bytes if no driver ever called netdev_rss_key_fill() function.
+File contains all nul bytes if no driver ever called netdev_rss_key_fill()
+function.
 
 Note:
-  /proc/sys/net/core/netdev_rss_key contains 52 bytes of key,
-  but most drivers only use 40 bytes of it.
+  /proc/sys/net/core/netdev_rss_key contains 256 bytes of key,
+  but many drivers only use 40 or 52 bytes of it.
 
 ::
 
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 179b50f98cee..d4e6e00bb90a 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5166,8 +5166,7 @@ void *netdev_lower_dev_get_private(struct net_device *dev,
 void netdev_lower_state_changed(struct net_device *lower_dev,
 				void *lower_state_info);
 
-/* RSS keys are 40 or 52 bytes long */
-#define NETDEV_RSS_KEY_LEN 52
+#define NETDEV_RSS_KEY_LEN 256
 extern u8 netdev_rss_key[NETDEV_RSS_KEY_LEN] __read_mostly;
 void netdev_rss_key_fill(void *buffer, size_t len);
 
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index 05dd55cf8b58..0f761d4b9447 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -325,10 +325,16 @@ static int proc_do_dev_weight(const struct ctl_table *table, int write,
 static int proc_do_rss_key(const struct ctl_table *table, int write,
 			   void *buffer, size_t *lenp, loff_t *ppos)
 {
-	struct ctl_table fake_table;
 	char buf[NETDEV_RSS_KEY_LEN * 3];
+	struct ctl_table fake_table;
+	char *pos = buf;
+
+	for (int i = 0; i < NETDEV_RSS_KEY_LEN; i++) {
+		pos = hex_byte_pack(pos, netdev_rss_key[i]);
+		*pos++ = ':';
+	}
+	*(--pos) = 0;
 
-	snprintf(buf, sizeof(buf), "%*phC", NETDEV_RSS_KEY_LEN, netdev_rss_key);
 	fake_table.data = buf;
 	fake_table.maxlen = sizeof(buf);
 	return proc_dostring(&fake_table, write, buffer, lenp, ppos);